Ammonium nitrate explosive composition containing vermicular low density expanded graphite



United States Patent 3,260,632 AMMONIUM NITRATE EXPLOSIVE CGMPUSHTION CONTAINENG VERMICULAR LOW DENSITY EXPANDED GRAPHITE Franciszek Olstowski, Freeport, Oliver Osborn, Lake Jackson, Frank D. Patrick, Freeport, and James R. Minderhout, Lake Jackson, Tern, assignors to The Dow Chemical Company, Midland, Micln, a corporation of Delaware No Drawing. Filed June 18, 1964, Ser. No. 376,241 7 Claims. (Cl. 149-43) This invention relates to explosives and more particularly is concerned with a novel ammonium nitrate based explosive having incorporated therein a vermicular, low density expanded graphite.

It is well recognized in the exposives art that inorganic based explosives employing ammonium nitrate as the principal ingredient are particularly suitable for use in mining, quarrying, excavating and the like blasting operations requiring large volumes of working gases and prolonged explosive work effort for the promotion of heave, thrust, movement and displacement of the material being blasted. Such ammonium nitrate based explosive compositions, however, generally are quite insensitive to initiation and require large amounts of a booster, i.e. shock-sensitive initiator, for detonation. Alternatively, large weight fractions of the total system of an easily detonable explosive such as tetryl, trinitrotoluene, nitroglycerine, etc. are used as a component or supplement in ammonium nitrate based explosives to impart initiation sensitivity into the mix. Use of such high explosives as a direct ingredient of the ammonium nitrate based composition is not entirely satisfactory as these components can change the entire character of the relatively safe to handle inorganic nitrate compositions and make these more difficult and dangerous to manufacture, handle, store and use.

It is a principal object of the present invention to provide a novel, readily detonable ammonium nitrate based explosive composition which does not contain a high explosive sensitizer.

It is another object of the present invention to provide a novel ammonium nitrate explosive composition employing a sensitizer which is not adversely affected by low temperatures.

It is a further object of the present invention to provide a novel, readily detonable ammonium nitrate explosive containing a liquid phase and a small amount of a nonexplosive solid sensitizer for the composition which is capable of immobilizing a large quantity of the liquid phase.

These and other objects and advantages readily will become apparent from the detailed description presented hereinafter.

The present invention comprises ammonium nitrate having in combination therewith, based on the weight of ammonium nitrate in composition, from about 0.1 to about 10 weight percent and preferably from about 1 to about 6 weight percent a vermicular, i.e. worm-like, expanded graphite. This graphite sensitizer employed in these explosive compositions has an apparent bulk density of from about 0.01 to about 10 pounds per cubic foot and preferably from about 0.1 to about 1 pound per cubic foot.

The resulting compositions readily are initiated by a small initiator, e.g. a 40 gram pentolite booster or a No. 6 or larger electric or fuse initiated blasting cap. Also Primacord of at least about 50 grains per foot can be used.

Preferably, in these ammonium nitrate-expanded graphite explosive compositions, the graphite sensitizer is substantially uniformly distributed throughout the mix.

Solid particulate ammonium nitrate, solutions having ice ammonium nitrate as a solute and ammonium nitrate containing slurries can be used with the expanded graphite to provide the present novel compositions.

With solid, particulate ammonium nitrate materials, the expanded graphite and ammonium nitrate are blended directly as by standard dry mixing, mix-mulling and other dry blending techniques. Since the expanded graphite has a low apparent bulk density, the resulting blend readily can be compressed, if desired, to provide an explosive composition of a predetermined density and configuration.

Because of the unusually high liquid sorbing characteristics of the expanded graphite with ammonium nitrate solutions and slurries, a predetermined amount of a pourable ammonium nitrate solution or slurry can be poured over the graphite or the graphite stirred or otherwise blended into the ammonium nitrate material to provide a composition of the desired degree of dryness or wetness. Usually, at a maximum, within the composition ranges set forth hereinbefore the amount of ammonium nitrate solution or slurry is such that it does not extend appreciably above the height of the expanded graphite in a cartridge, packaged explosive or other form of the present novel composition.

If desired, other fuel, oxidizer, sensitizing, and additive ingredients, e.g. particulate light metal fuels or sensitizers such as aluminum, magnesium, aluminum alloys, magnesium alloys and mixtures thereof, inorganic oxidizers such as sodium nitrate, as well as natural gums, gelling agents and the like thickening or suspending agents can also be incorporated into the present novel ammonium nitrate-expanded graphite explosive composition to provide tailor-made compositions for specific blasting utilities.

Generally with the light metal additives, for example, these are added at a maximum to provide a substantially fuel-oxidizer balanced composition.

Particulate ammonium nitrate products suitable for use in the present composition are fertilizer or explosive grade prills, granular, dense coarse or finely divided crystalline and the like materials.

Ammonium nitrate solutions employed ordinarily are saturated with respect to the ammonium nitrate solute although this is not critical since solutions less concentrated can be employed. Solvents for use in preparing the ammonium nitrate solutions can be any liquids which dissolve this component. Water, ammonia and aqueous ammonia solutions are particularly suitable solvents.

Divers liquid (ammonium nitrate dissolved in ammonia) and commercially available saturated solutions of ammonium nitrate in ammonia having from about 4 to 10 percent or more of water, preferably from about 4 to about 6 percent, on the total solution weight are especially effective.

The ammonium nitrate slurries used generally are composed of one or more of the solid particulate forms of ammonium nitrate set forth hereinbefore admixed with a solvent or carrier, e.g. water, ammonia, hydrazine, liquid hydrocarbon, fuel oil, crude oil, etc. for the particulate nitrate. With solvents, the relative amounts of the liquid and solid phases are such that there is at least some undissolvedammonium nitrate present along with the liquid phase. At a maximum, ordinarily the amount of solid phase is such that the resulting slurry is flowable.

With thicker liquid containing ammonium nitrate components such as thick slurries, pastes, dampened solid particulate materials etc. mix-mullers, dry-liquid blenders and other suitable mixers can be used to prepare a blend of the ammonium nitrate-expanded graphite.

The expanded graphite used in the present composition is prepared from particulate naturally occurring crystalline flake graphite and crystalline lump graphite, flake graphite being preferred. The particle size of graphite to be used is not critical although ordinarily particles of from about 10 to about 200 mesh US. Standard Sieve are used.

In preparing the expanded product, a particulate natural crystalline graphite is contacted at about room temperature with (1) a mixture of from about 8 to about 98 weight percent concentrated sulfuric acid (at least 90 percent H SO and from about 92 to about 2 weight percent concentrated nitric acid (at least about 60 weight percent HNO (2) fuming nitric acid, (3) fuming sulfuric acid, or (4) concentrated sulfuric acid (at least about 90 weight percent H 50 or concentrated nitric acid at least about 60 weight percent HNO plus at least about 2 weight percent of a solid inorganic oxidizer. The resulting =mix components usually are employed on a weight proportion basis of from about 0.22/1 (acid member/graphite). These are maintained in contact for at least about one minute, although a lengthy contact time of hours or days is not detrimental. The acid-treated graphite is separated from any excess acid, washed if desired, and subjected to rapid heating in a gaseous environment at a temperature of at least about 750 C.

Alternatively the graphite material can be similarly treated with an aqueous peroXy-halo acid, preferably perchloric or periodic acid, using an acid concentration of from about 2 to about 70 weight percent or more and an acid/ graphite Weight proportion of from about 0.05- 2/ 1, the acid treated graphite separated from excess acid and heated to a temperature of from about 180 to about 600 C. or higher.

The crystalline graphite also can be anodically electrolyzed in an aqueous acidic or aqueous salt electrolyte at an electrolyte temperature of from about to about 80 C. at a minimum cell potential of about 2 volts. The total quantity of electricity passed is equivalent to from about to about 500 ampere-hours per pound of graphite. The electrically treated graphite is separated from the electrolyte solution and rapidly heated at a temperature of at least about 150 The actual apparent bulk density of the final expanded product is determined in part by the temperature employed in the expansion operation. Satisfactory expansion of the aqueous peroxy halo-acid treated or electrolyzed crystalline material results at temperatures above about 150200 C. However, ordinarily a gaseous environment having a temperature of from about 500 to about 2000 C. or higher is used. Generally, as the temperature increases, the bulk density of the expanded product decreases. Ordinarily, the graphite products from all the acid treatments set forth hereinbefore are subjected to hydrocarbon fuel flames, e.g. propane torch (flame temperature about 1100 C.), oxyacetylene torch (flame temperature of about 1500 C. or higher) etc. for expansion. Generally, the acid-treated or electrolyzed graphite flake particulate material is placed in contact with the flame thereby to effect expansions of from 200 to 600 fold substantially instantaneously, e.g. within a second.

The time required for expansion also is dependent to a large extent on the heating temperature. Generally as the temperature rises, the time required for heating decreases. However, within the operable expansion temperature range set forth herein ordinarily the expansion is completed in less than a minute and a maximum heating period of five minutes has been found to be more than suflicient.

The expanded graphite resulting from this process is a vermicular, particulate product having a low apparent bulk density as set forth hereinbefore in comparison to the high density of crystalline graphite starting material. (To illustrate, a commercially available Madagascar flake graphite used as a starting material for preparing the expanded graphite product having a carbon content of greater than 80% and a nominal mesh size of from about 30 to about .50 11.8, Standard Sieve had an apparent bulk density of about 51.2 pounds per cubic foot.) The term apparent bulk density as used herein is the density determined from the volume occupied by a given mass of the product subjected to free fall (by gravity) into an open top container, e.g. a graduated cylinder.

The resulting particulate product readily can be compacted into any of a wide variety of integral shapes, loose or tight particulate compacts et cetera the density of which can be controlled by the amount of compacting.

The present invention is illustrated further by the following examples but is not meant to be limited thereto.

Example 1.-About 20 grams of a commercially available natural flake graphite (density about 51 pounds/ cubic foot) nominally of about 20-50 mesh (US. Standard Sieve) was contacted with a mixture of about 10 grams concentrated sulfuric acid (-98 weight percent H 50 and 5 grams of concentrated nitric acid (-60 weight percent HNO at room temperature for about 2 minutes.

The resulting acid treated flakes were separated from the acid, washed free of excess acid with water and dried.

The acid treated flakes were fed into the flame of a propane-air torch thereby being subjected to a temperature of about 1100 C. The flakes, upon contacting the flame, substantially instantaneously expanded into a wormlike particulate product having an apparent bulk density of about 0.6 pound per cubic foot.

An explosive composition was prepared using 900 cubic centimeters, i.e. about 900 grams, of an aqueous ammon'i-acal solution of ammonium nitrate (25.3 weight percent NH 69.2 weight percent NH NO and 5.5 weight percent H O) as oxidizer and about 20 grams of the expanded graphite. The graphite was placed in a 5 inch diameter 0.5 gallon capacity cylindrical paper carton and the ammonium nitrate solution poured over the graphite. This gave a final composition which was dry in appearance (i.e. contained substantially no visible free liquid) and had a density of about 0.87 gram per cubic centimeter.

The following compositions were prepared in similar containers and used as controls:

About 20 grams of a finely divided carbon black was suspended substantially homogeneously with the aid of a small amount of karaya gum in about 980 grams ammonium nitrate solution of the same composition.

About 70 grams of the natural flake graphite starting material used in the preparation of the expanded graphite product was similarly suspended in about 930 grams of the aqueous ammoniacal ammonium nitrate solution.

Each of the resulting compositions, plus a 1000 gram sample of the ammonium nitrate solution itself, was armed with a 40 gram pentolite booster placed on top of the charge. The booster in turn was initiated by a SO-gram per foot Primacord attached to a N0. 6 electric blasting cap.

The resulting compositions were subjected to a standard small lead block test to determine their detonability. This test, reported in Chemistry of Powder and Explosives, T. L. Davis, I. Wiley & Sons, New York (1941-43), p. 25, and The Science of High Explosives, M. A, Cook, ACS Monograph Series No. 139, Reinhold Publishing Corp., New York (1958), p. 34, provides a useful comparative evaluation of the brisance, i.e. shattering or fragmenting ability of a detonated explosive.

With the present compositions, for this test, the charge in the cylindrical paper container was placed on a 0.5 inch thick by 8 inch square steel plate which was centered over a 3.5 inch diameter by 3 inch high cylindrical lead block supported by a heavy steel anvil as a base. The deformation, i.e. reduction in height, of the lead block, expressed in thirty-seconds of an inch, is taken as a measure of the detonability and brisance of the charge.

Table I, which follows, summarizes the lead block deformation results obtained from the instant novel ammonium nitrate solution-expanded graphite explosive composition and the controls.

Table 1 All of the mixes were detonated while at the given temperature using the pentolite booster initiator and standard sensmm Deformati n lead block test procedure described in Example 1. The Free Carbon of Lead Run gg g g g (wt'fpement 3 g (in 5 results of these tests are reported in Table III.

ypeo at on 0 am- 211 's of monium inch) Table I nitrate) Lead None (control) 6 E 1 1 B1 k Carbon black (control). 3 8 Run position Temp. Deforma- Remarks Nilglxflfifiglahe Graphite 10.8 13 10 N0. (Sanitizer) 0 Q 3223 f s o Expanded Graphite 3.2 12 inch 1 Aluminum 28 88 Deformation only The results of these studies clearly show the desirable 9 r do minus 18 36 about 40% Shot and highly effective explosive detonation realized from a 15 at ambient temi t compositlon of the presentmvent on utilizing only a small 3 Em Graphite 28 72 Deformation about amount of expanded graphite sensitizer. 4 --do minus 18 58 81% of shot at Example 2.About grams of an expanded graphite @33 product similar to that described in Example 1 was dry blended with about 980 grams of prilled ammonium 2O nitrate. The resulting substantially homogeneous blend was compacted in a 0.5 gallon 5-inch diameter cylindrical paper container to provide a composition having a density of about 0.85 gram per cubic centimeter. The charge was armed in the same manner as described in Example 1 and detonated in a lead block test assembly. Deformation of the lead block was -inch.

As a control, about 70 grams of natural flake graphite, used as a starting material for the expanded product was blended with 930 grams ammonium nitrate and the resulting charge similarly armed and tested. lead block deformation was only about inch.

Example 3.-The expanded graphite sensitizer of the present invention was evaluated in comparison with flake aluminum (about 30 to about 90 mesh, US. Standard Sieve) as a sensitizer for the aqueous ammoniacal ammonium nitrate solution described in Example 1.-

In this study, explosive systems weighing about 1000 grams were prepared by admixing either the particulate aluminum or expanded graphite (similar to that described in Example 1) or mixtures of these materials with the ammonium nitrate solution in a 5 inch diameter paper carton. The resulting charges were armed and tested in accordance with the procedure described in Example 1.

The results of detonation studies from a number of compositions are presented in Table II which follows:

1 3.7-(Al) 0.15 (Exp. graphite).

Example 4.Two separate batches of 1000 gram explosive mixes were prepared containing 400 grams of flake aluminum (nominal mesh size -30 to +90 mesh, US. Standard Sieve) and 600 grams aqueous ammoniacal ammonium nitrate solution of composition substantially the same as described in Example 1.

Two 1000 gram batches of an explosive composition containing 980 grams of this same ammonium nitrate solution plus 20 grams of the expanded graphite also were prepared.

One of the mixes from each composition was cooled to an equilibrium temperature of minus 18 C. The other was maintained at ambient temperature (-28 C.).

The results of these tests show: (1) A small amount of expanded graphite (2% of total load weight) at ambient temperature gives results comparable to a highly metallized (40% of composition weight) explosive utilizing the same oxidizer solution and (2) at low temperatures there is a markedly better performance by the graphite sensitized composition. Not only is the reduction in effectiveness of the expanded graphite containing composition less at the lower temperatures (81% of higher temperature deformation for the graphite versus 40% of higher temperature deformation for the aluminum) but the actual deformation exhibited by the graphite sensitized mix at the low temperature is percent greater than the aluminum containing composition.

Example 5.--A thickened aqueous ammonium nitrate slurry explosive was prepared having on a weight basis the following nominal composition.

Parts by weight Ammonium nitrate 48 Water 11 Formamide 12 Aluminum scrap 12 Sodium nitrate 14 Expanded graphite 2 Guar gum 1 In the composition about 24 parts of the ammonium nitrate were in solution, the remainder being present as solid, crushed prills.

This composition was prepared as a 1000 gram charge and tested in the lead block test following the procedure described in Example 1. The resulting deformation was found to be inch.

Example 6.A 1000 gram explosive composition comprised of about 15 weight percent coarse aluminum lathe turnings 84.5 weight percent of aqueous ammoniacal ammonium nitrate solution of composition described in Example 1 and about 0.5 weight percent karaya gum as a thickening agent was found in the standard lead block test to have a deformation of about inch.

A similar charge was prepared except that 1 weight percent expanded graphite was blended into the mix. Detonation of this charge provided a lead block deformation of inch.

Example 7.About 10 grams of the expanded graphite was incorporated into a slurry explosive system containing about 500 grams of ammonium nitrate prills plus about 490 grams of a liquid solution consisting of methanol-ammonium nitrate-sodium nitrate-water in the weight ratio of about 60:267:34:129. The composition was tested in accordance with the standard lead block test described in Example 1. Lead block deformation was about inch.

As a control the slurry explosive itself without the expanded graphite was similarly tested. Lead block deformation was about inch.

In a manner similar to that described for the foregoing examples, vermicular expanded graphite having an apparent bulk density of from about 0.1 to about 10 pounds per cubic foot can be blended with ammonium nitrate containing explosives to provide readily detonable explosive compositions.

Although the low density expanded graphite product finds a particularly effective utility as a sensitizer for ordinarily relatively hard to detonate explosive materials it is to be understood that this same product can be used to upgrade the explosive performance of conventional explosives and oxidizer ingredients used in explosives. To illustrate, nitroglycerine, red fuming nitric acid, liquid oxygen can be sorbed by the expanded graphite to provide useful explosive compositions.

Various modifications can be made in the present invention without departing from the spirit or scope thereof for it is understood that we limit ourselves only as defined in the appended claims.

We claim:

1. An explosive composition which comprises ammonium nitrate and particulate, vermicular expanded graphite, said expanded graphite ranging from about 0.1 to about 10 weight percent of the weight of said ammonium nitrate and said expanded graphite having an apparent bulk density of from about 0.01 to about 10 pounds per cubic foot.

2. The explosive composition as defined in claim 1 wherein the amount of the expanded graphite ranges from about 1 to about 6 percent of the weight of said ammonium nitrate and has an apparent bulk density of from about 0.1 to 1 pound per cubic foot.

3. An explosive composition which comprises ammonium nitrate, vermicular expanded graphite and a particulate light metal, said expanded graphite ranging from about 0.1 to about 3 Weight percent of the weight of said ammonium nitrate, said expanded graphite having an apparent bulk density of from about 0.01 to about 10 pounds per cubic foot and said particulate light metal being present at a maximum so as to provide a substantially fueloxidizer balanced explosive composition.

4. The composition as defined in claim 1 wherein the ammonium nitrate is present as an ammoniacal solution, said solution being substantially saturated with respect to said ammonium nitrate.

5. The composition as defined in claim 1 wherein the ammonium nitrate is present as an aqueous solution, said solution being substantially saturated with respect to said ammonium nitrate.

6. The composition as defined in claim 1 wherein the ammonium nitrate is present as an aqueous ammoniacal solution, said solution being substantially saturated with respect to said ammonium nitrate and the water ranging from about 4 to about 10 percent on the weight of said solution.

7. The composition as defined in claim 3 wherein the ammonium nitrate is present as an aqueous solution, said solution being substantially saturated with respect to said ammonium nitrate.

References Cited by the Examiner UNITED STATES PATENTS 3,153,606 10/1964 Breza et al. 149-43 X LEON D. ROSDOL, Primary Examiner.

B. R. PADGETT, Assistant Examiner. 

3. AN EXPLOSIVE COMPOSITION WHICH COMPRISES AMMONIUM NITRATE, VERMICULAR EXPANDED GRAPHITE AND A PARTICULATE LIGHT METAL, SAID EXPANDED GRAPHITE RANGING FROM ABOUT 0.1 TIO ABOUT 3 WEIGHT PERCENT OF THE WEIGHT OF SAID AMMINIUM NITRATE, SAID EXPANDED GRAPHITE HAVING AN APPARENT BULK DENSITY OF FROM ABOUT 0.01 TO ABOUT 10 POUNDS PER CUBIC FOOT AND SAID PARTICULATE LIGHT METAL BEING PRESENT AT A MAXIMUM SO AS TO PROVIDE A SUBSTANTIALLY FUELOXIDIZER BALANCED EXPLOSIVE COMPOSITION. 